1. Field of the Invention
The present invention relates to a method for sampling a radio-frequency received signal which is down-mixed to a low-frequency baseband, in particular for the purposes of image production, such as the radio-frequency signal from a receiving coil in a magnetic resonance apparatus.
2. Description of the Prior Art
In known magnetic resonance systems, the radio-frequency signal which contains the measured patient information that is intended to be displayed as part of the magnetic resonance image, and which is picked up by a receiving coil, is down-mixed, by means of expensive and complex radio-frequency and analog circuits, to a baseband or to an intermediate frequency which is below the Nyquist limit of the sampler. This has been done in one stage, or generally in a number of stages. In addition, additional LO (local oscillator) carrier signals are required for each stage for this purpose, and these are derived from a mother oscillator, which is part of the complex circuit arrangement. In this case the LO frequencies have to be selected such that no mixing product is produced in the useful band. If any disturbing mixing products occurred despite this, they are removed by complex bandpass and low-pass filters, which result in further circuit complexity and costs.
After the radio-frequency signal which contains the useful information is down-mixed to baseband, the signal is digitized, such that the Nyquist criterion is satisfied, using a high-resolution and expensive analog/digital converter with an upstream low-pass filter, and is supplied directly to the further signal processing in order to produce the magnetic resonance image. The process of mixing down the radio-frequency received signal to an intermediate frequency followed by digitization means that digital demodulation is also required in order to change the signal to baseband. The Nyquist criterion once again had to be satisfied during digitization.
Overall, the sampling of radio-frequency received signals in the field of magnetic resonance apparatus is associated with a very high level of circuit complexity, which is extremely complicated and costly.
An object of the present invention is to provide a method for sampling, which allows reliable signal sampling with little circuit complexity.
In order to achieve this object using a method of the type initially described, the invention provides that the received radio-frequency signal is passed directly to a high clock rate analog/digital converter where it is converted, as a function of the clock, to a lower frequency, after which it is down-mixed by digital demodulation to a baseband at an even lower frequency.
In contrast to known methods, in the method according to the invention the received radio-frequency signal is not down-mixed to baseband before digitization but instead the received signal is passed directly to a high clock rate, broadband and high-speed analog/digital converter. The clock frequency of the analog/digital converter is such that, after conversion, the digital converter signal is in a frequency band which is lower than the original signal frequency. With regard to the sampling frequency, all that need be borne in mind is to ensure that the sampling is below the Nyquist criterion, while at the same time complying with the extended Nyquist criterion. The capability to apply the radio-frequency received signal directly to the analog/digital converter is a result of the fact that converters have now become available which operate with very broad bandwidths and have extremely low noise levels, with a very high clock frequency at the same time. This converter generation allows even high received signal frequencies to be processed, such as those in the signal received from a magnetic resonance apparatus where, depending on the system design, signals may be received even at frequencies above 60 MHZ.
According to the Nyquist criterion, a necessary condition for reproducibility of the received signal is that the sampling frequency is at least twice the signal frequency. The sampling process results in the frequency packet being repeated more than once, cyclically and as a function of the sampling frequency, in the frequency band extending from xe2x88x92∞ to +∞. If the sampling frequency is below the Nyquist criterion, the signals become superimposed thus resulting in xe2x80x9cmixing productsxe2x80x9d which make it more complicated to reproduce the received signal after digitization. However, if the sampling frequency is at or above the Nyquist criterion, then reproducibility is feasible, since there are no superimposition effects.
According to the extended Nyquist criterion, a sufficient condition for reproducible sampling is for the sampling frequency to be at least twice the bandwidth of the useful signal, in this case of the signal which is received from the receiving coil and carries the useful information. If the signal bandwidth, which is limited by previous filtering, is, for example, xc2x1250 kHz, then the sampling frequency must be at least 1 MHZ. Thus, if a limited bandwidth signal is present, it is generally sufficient for the sampling to comply only with the extended Nyquist criterion, i.e., sampling can be carried out using a frequency below twice the signal frequency, provided it corresponds at least to twice the bandwidth of the signal.
Since the received signal carrying the useful information required for image processing has a relatively narrow bandwidth, it is possible for magnetic resonance system to use sampling frequencies which are well below the Nyquist criterion.
The digital converter signal, whose frequency is already well below that of the received signal, is then digitally demodulated and is down-mixed to the low-frequency baseband. Any suitable known demodulation method may be used for demodulation.
Overall, the method according to the invention allows reliable sampling with considerably less circuit complexity since there is no longer any need for complex and expensive radio-frequency and analog circuits, which are provided in the prior art for down-mixing the radio-frequency received signal. As a consequence, it is possible to keep the sampling circuit arrangement very small, and to integrate it at a suitable point on already existing circuit boards. Conversely, it is possible to integrate the analog/digital converter directly into the coil so that, in this case, the converter signal, which is already at a lower frequency, is passed to the downstream demodulation stage, which is external to the coil.
In order to reduce the amount of noise in the received radio-frequency signal, it is expedient to filter the received signal in a bandpass filter before passing it to the analog/digital converter. This filter, which expediently filters the received signal to a bandwidth in the range between xc2x1100 kHz to xc2x1500 kHz, in particular of xc2x1250 kHz, about the received signal frequency, makes it possible to filter out those frequencies which are above and below this band and do not contain any useful signal, but only noise, so that the converter need process only the filtered received signal, which contains considerably less noise.
Furthermore, it is advantageous for the received signal to be amplified before being passed to the analog/digital converter and, expediently, before the previous filtering. This amplification allows the signal to be matched to the performance parameters of the analog/digital converter so that it can be driven at an appropriate level.
A multiplication stage is expediently used for demodulation. Furthermore, data reduction can be carried out in a digital low-pass filter stage after demodulation, since, because of the high sampling frequency, which may be in the order of magnitude of several tens of megahertz, the amount of data is too great for the subsequent signal processing, and a large amount of data of this type is also not required for image production.
In addition to the method according to the invention, the invention also relates to a magnetic resonance apparatus having a circuit arrangement for sampling a radio-frequency received signal from a receiving coil, with the circuit arrangement having:
a high clock rate analog/digital converter, to which the radio-frequency received signal can be passed directly, for clock-dependent conversion of the radio-frequency received signal to a lower frequency, and
a digital demodulation device, which is connected downstream from the analog/digital converter, for down-mixing the digital output signal from the analog/digital converter to a lower-frequency baseband.
According to the invention, a bandpass filter for limiting the received signal to a specific bandwidth may be connected upstream of the analog/digital converter, preferably using a bandwidth in the range from xc2x1100 kHz to xc2x1500 kHz, in particular of xc2x1250 kHz. An amplifier device for amplifying the received signal may be connected upstream of the analog/digital converter, with this amplifier device being designed such that the incoming received signal is designed for optimum matching of the signal to the performance parameters of the analog/digital converter. Known analog/digital converters require a signal amplitude of approximately 1 V while, in contrast, the receiving coil produces signals in the order of magnitude of approximately 0.1 V.
Furthermore, it is possible to provide for the demodulation device to have a multiplication stage, with at least one low-pass filter stage expediently being connected downstream from the demodulation device for data reduction. Finally, the analog/digital converter may be integrated directly in the receiving coil.